Package aptitude-doc: Information

- Untied from the details of an interface in libapt by re-using a class from
  libapt (>= 0.5.15lorg2-alt86). (The dep was due to us reimplementing it.)

- Fixed the crash of "aptitude download" (with a large enough total
  number of known packages, like in ALT Sisyphus).

- Fixed build by including a necessary header
  (an error that surfaced with gcc-12.2.1-alt1).